Changing the frequency or phase of the switching clock driving the pulse width modulator (PWM) in a class-D amplifier is known to produce artifacts at the output, which manifest as audible clicks. These clicks are known as pop-noise, which are an increasingly important parameter for audio amplifiers.
There are several reasons that the frequency or phase of the class-D switching clock might be changed. These include the use of frequency hopping to avoid the amplifier output switching frequency or a harmonic being on a tuned AM frequency; spread spectrum modulation in which the PWM switching clock frequency is modulated with a slowly varying signal to broaden the switching spectrum and so reduce the spectral density of electromagnetic interference (EMI); synchronisation of several Class-D amplifier integrated circuits in a system so that they all operate in phase; and changing between AD modulation (in which the half-bridges of the output channel are switched out of phase with each other) and BD modulation (in which the half-bridges are switched in phase).
To measure the pop-noise produced by a Class-D audio amplifier, the differential output voltage of the audio amplifier is first converted into a single-ended voltage. This single-ended voltage is fed through a 20 kHz low-pass filter and an ITU-R 2 k filter. Finally, the pop-noise is measured as the peak output voltage of the ITU-R 2 k filter. Around 5 years ago 10 mV pop-noise would have been considered perfectly acceptable; today 3 mV pop-noise is demanded; and it is expected that in the future lower values will be demanded.